draft-link-dhc-v6only-00.txt   draft-link-dhc-v6only-01.txt 
Dynamic Host Configuration L. Colitti Dynamic Host Configuration L. Colitti
Internet-Draft J. Linkova Internet-Draft J. Linkova
Intended status: Standards Track Google Intended status: Standards Track Google
Expires: June 6, 2020 M. Richardson Expires: June 11, 2020 M. Richardson
Sandelman Sandelman
T. Mrugalski T. Mrugalski
ISC ISC
December 4, 2019 December 9, 2019
IPv6-Only-Preferred Option for DHCP IPv6-Only-Preferred Option for DHCP
draft-link-dhc-v6only-00 draft-link-dhc-v6only-01
Abstract Abstract
This document specifies a DHCP option to indicate that a host This document specifies a DHCP option to indicate that a host
supports an IPv6-only mode and willing to forgo obtaining a IPv4 supports an IPv6-only mode and willing to forgo obtaining an IPv4
address if the network provides IPv6 access. address if the network provides IPv6 connectivity.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 6, 2020. This Internet-Draft will expire on June 11, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 13 skipping to change at page 2, line 13
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets . . . . 4 2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets . . . . 4
3. IPv6-Only Preferred Option . . . . . . . . . . . . . . . . . 4 3. IPv6-Only Preferred Option . . . . . . . . . . . . . . . . . 5
3.1. Option format . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Option format . . . . . . . . . . . . . . . . . . . . . . 5
3.2. DHCPv4 Client Behaviour . . . . . . . . . . . . . . . . . 5 3.2. DHCPv4 Client Behaviour . . . . . . . . . . . . . . . . . 5
3.3. DHCPv4 Server Behaviour . . . . . . . . . . . . . . . . . 6 3.3. DHCPv4 Server Behaviour . . . . . . . . . . . . . . . . . 7
3.4. Configuration Variables . . . . . . . . . . . . . . . . . 7 3.4. Configuration Variables . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8 7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 8 7.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction 1. Introduction
One of the biggest challenges of deploying IPv6-only LANs is that One of the biggest challenges of deploying IPv6-only LANs is that
such networks might contain rather heterogeneous collection of end such networks might contain rather heterogeneous collection of hosts.
hosts. Some of them are capable of operating in IPv6-only mode Some of them are capable of operating in IPv6-only mode (either
(either because the OS and all applications are IPv6-only capable or because the OS and all applications are IPv6-only capable or because
because the host has some form of 464XLAT [RFC6877] deployed). At the host has some form of 464XLAT [RFC6877] deployed). At the same
the same time some devices might still have IPv4 dependencies and time some devices might still have IPv4 dependencies and need IPv4
need IPv4 connectivity to operate properly. To incrementaly rollout connectivity to operate properly. To incrementally rollout
IPv6-only, network operators need to provide IPv4-as-a-service when a IPv6-only, network operators need to provide IPv4-as-a-service
host receives an IPv4 address if it needs it, while IPv6-only capable whereby a host receives an IPv4 address if it needs it, while
devices (such as modern mobile devices) are not allocated IPv4 IPv6-only capable devices (such as modern mobile devices) are not
addresses. Deploying separate LAN segments for IPv6-only and for allocated IPv4 addresses. Deploying separate LAN segments for
dual-stack hosts (such as two WiFi SSIDs or two VLANs) is undesirable IPv6-only and for dual-stack hosts (such as two WiFi SSIDs or two
for a number of reasons, including but not limited to: VLANs) is undesirable for a number of reasons, including but not
limited to:
o Doubling number of network segments which leads to operational o Doubling the number of network segments which leads to operational
complexity and performance impact, for instance due to TCAM complexity and performance impact, for instance due to TCAM
utilization increase from an increased number of ACL entries. utilization increase from an increased number of ACL entries.
o Placing a host into correct network segment is problematic. For o Placing a host into the correct network segment is problematic.
example, in the case of 802.11 Wi-Fi the user might select the For example, in the case of 802.11 Wi-Fi the user might select the
wrong SSID. In the case of wired 802.1x authentication the wrong SSID. In the case of wired 802.1x authentication the
authentication server might not have all information required to authentication server might not have all the information required
make the correct decision. to make the correct decision.
Therefore it would be beneficial for IPv6 deployment if operators It would be beneficial for IPv6 deployment if operators could
could implement IPv4-mostly (or IPv4-as-a-Service) segments where implement IPv6-mostly (or IPv4-as-a-Service) segments where IPv6-only
IPv6-only hosts co-exist with legacy dual-stack devices. The trivial hosts co-exist with legacy dual-stack devices. The trivial solution
solution of disabling IPv4 stack on IPv6-only capable hosts is not of disabling IPv4 stack on IPv6-only capable hosts is not feasible as
feasible as those clients must be able to operate on IPv4-only those clients must be able to operate on IPv4-only networks as well.
networks as well. While IPv6-only capable devices might use a While IPv6-only capable devices might use a heuristic approach to
heuristic approach to learning if the network provdes IPv6-only learning if the network provides IPv6-only functionality and stop
functionality and stop using IPv4 if it does, it might be practically using IPv4 if it does, it might be practically undesirable. One
undesirable. One important reason is that when a host connects to a important reason is that when a host connects to a network, it does
network, it does not know if the network is IPv4-only, dual-stack or not know if the network is IPv4-only, dual-stack or IPv6-only. To
IPv6-only. To ensure that the connectivity over whatever protocol is ensure that the connectivity over whatever protocol is present
present becomes available as soon as possible the host usually starts becomes available as soon as possible the host usually starts
configuring both IPv4 and IPv6 immidiately. If hosts were to delay configuring both IPv4 and IPv6 immediately. If hosts were to delay
requesting IPv4 until IPv6 reachability is confirmed, that would requesting IPv4 until IPv6 reachability is confirmed, that would
would penalize IPv4-only and dual-stack networks, which does not seem penalize IPv4-only and dual-stack networks, which does not seem
practical. Instead it would be useful to have a mechanism which practical. Requesting IPv4 and then releasing it later, after IPv6
would allow a host to indicate that IPv4 is optional and a network to reachability is confirmed, might cause user-visible errors as it
signal that IPv6-only functionality (such as NAT64) is available. would be disruptive for applications which have started using the
The proposed solution is to introduce a new DHCP option which a assigned IPv4 address already. Instead it would be useful to have a
client uses to indicate that it does not need IPv4 if the network mechanism which would allow a host to indicate that IPv4 is optional
provides IPv6-only connectivty (as NAT64 and DNS64). If the and a network to signal that IPv6-only functionality (such as NAT64)
particular network segment provides IPv4-as-a-service such clients is available. The proposed solution is to introduce a new DHCP
would not be supplied with IPv4 addresses, while on IPv4-only or option which a client uses to indicate that it does not need IPv4 if
dual-stack segments without NAT64 services IPv4 addresses will be the network provides IPv6-only connectivity (as NAT64 and DNS64). If
the particular network segment provides IPv4-as-a-service such
clients would not be supplied with IPv4 addresses, while on IPv4-only
or dual-stack segments without NAT64 services IPv4 addresses will be
provided. provided.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
1.2. Terminology 1.2. Terminology
IPv6-only capable host: a host which does not require IPv4 and can IPv6-only capable host: a host which does not require IPv4 and can
operate on IPv6-only networks. Strictly speaking IPv6-only operate on IPv6-only networks. Strictly speaking IPv6-only
capability is specific to a given interface of the host: if some capability is specific to a given interface of the host: if some
applocation on a host require IPv4 and 464XLAT clat [RFC6877] is only application on a host require IPv4 and 464XLAT CLAT [RFC6877] is only
enabled on one interface, the host is IPv6-only capable if connected enabled on one interface, the host is IPv6-only capable if connected
to a NAT64 network via that interface. to a NAT64 network via that interface.
IPv4-as-a-Service: a deployment scenario when end hosts are expected IPv4-as-a-Service: a deployment scenario when end hosts are expected
to operate in IPv6-only mode by default and IPv4 addresses can be to operate in IPv6-only mode by default and IPv4 addresses can be
assigned to some hosts if those hosts explicitly opt-in to receiving assigned to some hosts if those hosts explicitly opt-in to receiving
IPv4 addresses; IPv4 addresses.
IPv6-mostly network: a network which provides NAT64 (possibly with IPv6-mostly network: a network which provides NAT64 (possibly with
DNS64) service as well as IPv4 connectivity. Such deployment DNS64) service as well as IPv4 connectivity. Such deployment
scenario allows operators to incrementaly turn off IPv4 on end hosts, scenario allows operators to incrementally turn off IPv4 on end
while still providing IPv4 to devices which require IPv4 to operate. hosts, while still providing IPv4 to devices which require IPv4 to
But, IPv6-only capable devices need not be assigned IPv4 addresses. operate. But, IPv6-only capable devices need not be assigned IPv4
addresses.
IPv6-Only network: a network which does not provide routing IPv6-Only network: a network which does not provide routing
functionality for IPv4 packets. Such network may or may not allow functionality for IPv4 packets. Such networks may or may not allow
intra-LAN IPv4 connectivity. IPv6-Only network usually provide intra-LAN IPv4 connectivity. IPv6-Only network usually provides
access to IPv4-only resources via NAT64 [RFC6147]. access to IPv4-only resources via NAT64 [RFC6147].
NAT64: Network Address and Protocol Translation from IPv6 Clients to NAT64: Network Address and Protocol Translation from IPv6 Clients to
IPv4 Servers [RFC6146]; IPv4 Servers [RFC6146];
RA: Router Advertisement, a message used by IPv6 routers to advertise RA: Router Advertisement, a message used by IPv6 routers to advertise
their presence together with various link and Internet parameters their presence together with various link and Internet parameters
[RFC4861]; [RFC4861];
DNS64: a mechanism for synthesizing AAAA records from A records DNS64: a mechanism for synthesizing AAAA records from A records
[RFC6147]; [RFC6147];
2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets 2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets
For networks which contains both IPv6-capable and IPv4-requiring For networks which contain both IPv6-capable and IPv4-requiring
devices and utilizes DHCP for configuring IPv4 network stack on devices and utilize DHCP for configuring IPv4 network stack on hosts,
hosts, it seems only natural to leverage the same protocol to signal it seems only natural to leverage the same protocol to signal that
that IPv4 is discretional on the given segment. Such approach limits IPv4 is discretional on a given segment. Such an approach limits the
the attack surface to DHCP-related attacks without introducing new attack surface to DHCP-related attacks without introducing new
vulnerable elements. vulnerable elements.
Another benefit of using DHCPv4 for signalling is that IPv4 will be Another benefit of using DHCPv4 for signaling is that IPv4 will be
disabled only if both the client and the server indicate IPv6-only disabled only if both the client and the server indicate IPv6-only
capability. It allows IPv6-only capable clients to turn off IPv6 capability. It allows IPv6-only capable hosts to turn off IPv4 only
only upon receiving an explicit signal from the network and operate upon receiving an explicit signal from the network and operate in
in dual-stack or IPv4-only mode otherwise. dual-stack or IPv4-only mode otherwise.
Coexistence of IPv6-only, dual-stack and even IPv4-only hosts on the Coexistence of IPv6-only, dual-stack and even IPv4-only hosts on the
same LAN would not only allow network administrators to preserve same LAN would not only allow network administrators to preserve
scarce IPv4 addresses but would also drastically simplify incremental scarce IPv4 addresses but would also drastically simplify incremental
deployment of IPv6-only networks, positively impacting IPv6 adoption. deployment of IPv6-only networks, positively impacting IPv6 adoption.
3. IPv6-Only Preferred Option 3. IPv6-Only Preferred Option
3.1. Option format 3.1. Option format
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 0 1 2 3
| Type | Length | 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (contd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IPv6-Only Preferred Option Format Figure 1: IPv6-Only Preferred Option Format
Fields: Fields:
Type 8-bit identifier of the IPv6-Only Preferred option type as Type 8-bit identifier of the IPv6-Only Preferred option type as
assigned by IANA: TBD assigned by IANA: TBD
Length 8-bit unsigned integer. The length of the option (excluding Length 8-bit unsigned integer. The length of the option excluding
the Type and Length Fields. The server MUST set the length the Type and Length Fields. The server MUST set the length
field to 0. The receiver MUST ignore the IPv6-Only Preferred field to 4. The receiver MUST ignore the IPv6-Only Preferred
option if the the length field value is not 0. option if the length field value is not 4.
Value 32-bit unsigned
integer. The number of
seconds the client should disable DHCPv4 for (V6ONLY_WAIT
configuration variable).
If the server pool is explicitly conifgured with V6ONLY_WAIT
timer the server MUST set the field to that configured value.
Otherwise the server MUST set it to zero.
The client MUST ignore V6ONLY_WAIT timer received from the
server if the value is less than 300 seconds.
3.2. DHCPv4 Client Behaviour 3.2. DHCPv4 Client Behaviour
A DHCP client SHOULD allow a device administrator to configure A DHCP client SHOULD allow a device administrator to configure
IPv6-only preferred mode either for a specific interface (to indicate IPv6-only preferred mode either for a specific interface (to indicate
that the device is IPv6-only capable if connected to a NAT64 network that the device is IPv6-only capable if connected to a NAT64 network
via that interface) or for all interfaces. If only a specific via that interface) or for all interfaces. If only a specific
interface is configured as IPv6-only capable the DHCP client MUST NOT interface is configured as IPv6-only capable the DHCP client MUST NOT
be considered as an IPv6-capable for the purpose of sending/receiving be considered as an IPv6-capable for the purpose of sending/receiving
DHCP packets over any other interfaces. DHCP packets over any other interfaces.
Clients not capable of operating in an IPv6-only NAT64 environment Clients not capable of operating in an IPv6-only NAT64 environment
MUST NOT include the IPv6-only Preferred option in the Parameter MUST NOT include the IPv6-only Preferred option in the Parameter
Request List of any DHCP packets and MUST ignore that option in Request List of any DHCP packets and MUST ignore that option in
packets received from DHCP servers. packets received from DHCP servers.
IPv6-only capable clients SHOULD include the IPv6-only Preferred IPv6-only capable clients SHOULD include the IPv6-only Preferred
option in the Parameter Request List in DHCPDISCOVER and DHCPREQUEST option in the Parameter Request List in DHCPDISCOVER and DHCPREQUEST
messages. messages for interfaces so enabled and follow the processing as
described below on a per interface enabled basis.
If the client did not include the IPv6-only Preferred option in the If the client did not include the IPv6-only Preferred option in the
DHCPDISCOVER or DHCPREQUEST message it MUST ignore the the IPv6-only DHCPDISCOVER or DHCPREQUEST message it MUST ignore the IPv6-only
Preferred option in any messages received from the server. Preferred option in any messages received from the server.
If the client includes the IPv6-only Preferred option in the If the client includes the IPv6-only Preferred option in the
Parameter Request List and the DHCPOFFER message from the server Parameter Request List and the DHCPOFFER message from the server
contains a valid IPv6-only Preferred option, the client MUST NOT contains a valid IPv6-only Preferred option, the client MUST NOT
configure the IPv4 address provided in the DHCPOFFER. The client configure the IPv4 address provided in the DHCPOFFER. If the
SHOULD stop the DHCP configuration process for at least V6ONLY_WAIT IPv6-only Preferred option returned by the server contains non-zero
seconds or until a network attachement event happens. The host MAY value the client SHOULD set the V6ONLY_WAIT timer to that value. If
disable IPv4 stack completely for V6ONLY_WAIT seconds or until the the server returns zero value the client MUST use its own
network disconnection event heppens. configuration for V6ONLY_WAIT timer. The client SHOULD stop the DHCP
configuration process for at least V6ONLY_WAIT seconds or until a
network attachment event happens. The host MAY disable IPv4 stack
completely for V6ONLY_WAIT seconds or until the network disconnection
event happens.
The client SHOULD include the IPv6-only Preferred option in The client SHOULD include the IPv6-only Preferred option in
DHCPREQUEST messages (after receiving a DHCPOFFER without this DHCPREQUEST messages (after receiving a DHCPOFFER without this
option, for a INIT-REBOOT, or when renewing or rebinding a leased option, for a INIT-REBOOT, or when renewing or rebinding a leased
address). If the DHCP server responds with a DHCPACK that includes address). If the DHCP server responds with a DHCPACK that includes
the IPv6-only Preferred option, the client MAY send a DHCPRELEASE the IPv6-only Preferred option, the client MAY send a DHCPRELEASE
message and MAY either stop the DHCP configuration process or disable message and MAY either stop the DHCP configuration process or disable
IPv4 stack completely for V6ONLY_WAIT seconds or until the network IPv4 stack completely for V6ONLY_WAIT seconds or until the network
disconnection event heppens. Alternatively the client MAY continue disconnection event happens. Alternatively the client MAY continue
to use the assigned IPv4 address until further DHCP reconfiguration to use the assigned IPv4 address until further DHCP reconfiguration
events. events.
If the client includes the IPv6-only Preferred option in the If the client includes the IPv6-only Preferred option in the
Parameter Request List and the server responds with DHCPOFFER message Parameter Request List and the server responds with DHCPOFFER message
without a valid IPv6-only Preferred option, the client MUST proceed without a valid IPv6-only Preferred option, the client MUST proceed
as normal with a DHCPREQUEST. as normal with a DHCPREQUEST.
If the client waits for multiple responses and the server the client If the client waits for multiple DHCPOFFER responses and selects one
sends the DHCPREQUEST to did not include the IPv6-only Preferred of them, it MUST follow the processing for the IPv6-only Preferred
option in the DHCPOFFER, the client MUST NOT stop the DHCP option based on the selected response. A client MAY use the presence
configuration process or disable IPv4 stack even if other servers of the IPv6-only Preferred option as a selection criteria.
include the IPv6-only Preferred option in their responses.
When an IPv6-only capable client receives the IPv6-Only Preferred When an IPv6-only capable client receives the IPv6-Only Preferred
option from the server, The client MAY configure IPv4 link-local option from the server, the client MAY configure IPv4 link-local
address [RFC3927]. In that case IPv6-Only capable devices might address [RFC3927]. In that case IPv6-Only capable devices might
still be able to communicate over IPv4 to other devices on the link. still be able to communicate over IPv4 to other devices on the link.
3.3. DHCPv4 Server Behaviour 3.3. DHCPv4 Server Behaviour
The DHCP server SHOULD have a configuration option to mark the given The DHCP server SHOULD have a configuration option to configure the
DHCP pool as belonging to an IPv6-mostly network segment. given DHCP pool with an IPv6-only preferred option. The DHCP server
MAY have a configuration option to specify V6ONLY_WAIT timer for all
or individual IPv6-mostly pools.
The server MUST NOT include the IPv6-only Preferred option in the The server MUST NOT include the IPv6-only Preferred option in the
DHCPOFFER or DHCPACK message if the YIADDR field in the message does DHCPOFFER or DHCPACK message if the YIADDR field in the message does
not belong to a pool configured as IPv6-mostly. The server MUST NOT not belong to a pool configured as IPv6-mostly. The server MUST NOT
include the IPv6-only Preferred option in the DHCPOFFER or DHCPACK include the IPv6-only Preferred option in the DHCPOFFER or DHCPACK
message if the option was not present in the Parameter Request List message if the option was not present in the Parameter Request List
sent by the client. sent by the client.
If the IPv6-only Preferred option is present in the Parameter Request If the IPv6-only Preferred option is present in the Parameter Request
List received from the client and the corresponding DHCP pool is List received from the client and the corresponding DHCP pool is
explicitly configured as belonging to an IPv6-mostly network segment, explicitly configured as belonging to an IPv6-mostly network segment,
the server MUST include respond with the DHPCOFFER or DHCPACK the server MUST include the IPv6-only Preferred option when
message. If the pool is explicitly configured with a dedicated IPv4 responding with the DHCPOFFER or DHCPACK message. If the server
address to be returned to IPv6-only capable clients the server MUST responds with the IPv6-only Preferred option and the V6ONLY_WAIT
specify that address as the client's network address and MUST NOT timer is configured for the pool, the server MUST copy the configured
verify its uniqueness. Otherwise the server SHOULD follow the value to the IPv6-only Preferred option value field. Otherwise it
recommendations in [RFC2131]. The client is not expected to use that MUST set the field to zero. The server SHOULD include an available
IPv4 address so if the client responds with the DHCPREQUEST message IPv4 address from the pool into the DHCPOFFER as per recommendations
for that address the server SHOULD respond with DHCPNAK. in [RFC2131] but SHOULD NOT reserve the address and SHOULD NOT verify
its uniqueness. The client is not expected to use that IPv4 address
so if the client responds with the DHCPREQUEST message for that
address the server SHOULD respond with DHCPNAK.
As an optional optimization an IPv6-mostly pool MAY be configured
with a dedicated IPv4 address to be returned to IPv6-only capable
clients. In that case the server SHOULD specify that address as the
client's network address and MUST NOT verify its uniqueness.
If a client includes both a Rapid-Commit option [RFC4039] and If a client includes both a Rapid-Commit option [RFC4039] and
IPv6-Only Preferred option in the DHCPDISCOVER message the server IPv6-Only Preferred option in the DHCPDISCOVER message the server
SHOULD NOT honor the Rapid-Commit option if the response woul contain SHOULD NOT honor the Rapid-Commit option if the response would
the IPv6-only Preferred option to the client. It SHOULD instead contain the IPv6-only Preferred option to the client. It SHOULD
respond with a DHCPOFFER so that the IP address does not need to be instead respond with a DHCPOFFER so that the IP address does not need
reserved for the client until the lease expires. to be reserved for the client until the lease expires.
3.4. Configuration Variables 3.4. Configuration Variables
V6ONLY_WAIT The minimum time the client SHOULD stop the DHCP V6ONLY_WAIT The minimum time the client SHOULD stop the DHCP
configuration process for. MUST be no less than 300 configuration process for. MUST be no less than 300
seconds. Default: 1800 seconds seconds. Default: 1800 seconds
4. IANA Considerations 4. IANA Considerations
The IANA is requested to assign a new DHCP Option code for the The IANA is requested to assign a new DHCP Option code for the
skipping to change at page 7, line 40 skipping to change at page 8, line 31
+----------------------------+-------+ +----------------------------+-------+
| IPv6-only Preferred option | (TBD) | | IPv6-only Preferred option | (TBD) |
+----------------------------+-------+ +----------------------------+-------+
Table 1 Table 1
5. Security Considerations 5. Security Considerations
The proposed mechanism is not introducing any new security The proposed mechanism is not introducing any new security
implications. While clients using the IPv6-only Preferred option are implications. While clients using the IPv6-only Preferred option are
vunerable to attacks related to a rogue DHCP server, enabling vulnerable to attacks related to a rogue DHCP server, enabling
IPv6-only Preferred option does not provide an attacker with any IPv6-only Preferred option does not provide an attacker with any
additional mechanisms. additional mechanisms.
It should be noted that disabling IPv4 on a host upon receiving the It should be noted that disabling IPv4 on a host upon receiving the
IPv6-only Preferred option from the DHCP server protects the host IPv6-only Preferred option from the DHCP server protects the host
from IPv4-related attacks and therefore could be considered a from IPv4-related attacks and therefore could be considered a
security feature. security feature.
6. Acknowledgements 6. Acknowledgements
Thanks to the following people (in alphabetical order) for their Thanks to the following people (in alphabetical order) for their
review and feedback: Bernie Volz. review and feedback: Mohamed Bboucadair, Bjorn Mork, Bernie Volz (AI:
add more names here). Authors would like to thank Bob Hinden and
Brian Carpenter for the initial idea of signaling IPv6-only
capability to hosts.
7. References 7. References
7.1. Normative References 7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", [RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997, RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>. <https://www.rfc-editor.org/info/rfc2131>.
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